The present invention is directed to novel benzo[b][1,8]-naphthyridine derivatives of general formula (I): 
to their salts, to their preparation and to the compositions which comprise them.
Benzonaphthyridine derivatives with the structure: 
in which R1 is H, hydroxyl or alkyl, R2 is H, alkyl, fluoroalkyl, cycloalkyl, alkyloxy or alkylamino, R3 is optionally substituted phenyl or phenylalkyl, and R4 is H or a fluorine atom, have been disclosed in European Patent Application EP 431,991. These compounds are of use as antimicrobial agents.
It has now been found that the benzonaphthyridine derivatives of general formula (I) described below, as well as their salts and, when appropriate, their stereoisomers, display an antibacterial activity which is advantageous for topical administration. These benzonaphthyridine derivatives belong to the group of compounds of general formula (I) in which:
R1 and R2, which are identical or different, represent an alkyl or cycloalkyl radical comprising 3 to 8 members, or form, together with the nitrogen atom to which they are attached, a 6-membered heterocycle, optionally comprising another nitrogen atom, which can be substituted at the 4-position by a phenyl radical, a substituted phenyl radical (substituted by a halogen atom or by an alkylthiomethyl radical), a benzyl radical, a benzyl radical substituted by a halogen atom, a heterocyclylmethyl radical, the heterocyclyl part of which is saturated or unsaturated and comprises 5 or 6 members, a phenylamino radical or a phenylamino radical, the phenyl part of which is optionally substituted by a halogen atom, or R1 and R2 form, together with the nitrogen atom to which they are attached, a 7- or 8-membered heterocycle, optionally comprising another nitrogen atom, which can be substituted by one of the substituents of the 6-membered heterocycle listed above;
R3 represents an alkyl radical, a fluoroalkyl radical, a carboxyalkyl radical, a cycloalkyl radical comprising 3 to 6 carbon atoms, a fluorophenyl radical, a difluorophenyl radical, an alkyloxy radical or an alkylamino radical; and
R4 represents a hydrogen atom or a fluorine atom;
the abovementioned alkyl radicals being straight or branched and comprising 1 to 4 carbon atoms.
In the above general formula, the halogen substituents can be chosen from chlorine, fluorine, bromine and iodine and the heterocyclyl radicals can be chosen from furyl, thienyl, pyridyl, pyrimidyl, imidazolyl, thiazolyl and pyrrolidyl. Furthermore, when R1 and R2 form, together with the nitrogen atom to which they are attached, a 6- to 8-membered heterocycle, the latter can be chosen without implied limitation from piperidine, piperazine, perhydroazepine, perhydroazocine, perhydrodiazepine, morpholine and thiomorpholine.
According to the invention, the compounds of general formula (I) can be obtained by substitution of an amine of general formula (II):
R1xe2x80x94NHxe2x80x94R2xe2x80x83xe2x80x83(II) 
in which R1 and R2 are defined as above, with a benzo[b][1,8]naphythridine of general formula (III): 
in which R3 is defined as above, Hal is a fluorine, chlorine or bromine atom if R4 is hydrogen, or else Hal and R4 are simultaneously fluorine atoms.
The reaction of an amine of general formula (II) with the benzonaphythridine derivative is generally carried out in the presence of an excess of the benzonaphythridine derivative of general formula (III) as acid acceptor in suitable organic solvents. It is possible to carry out the reaction with or without solvent, at a temperature ranging from approximately 20 to approximately 150xc2x0 C. When the reaction is carried out in the presence of a solvent, it can be carried out in solvents such as pyridine, dimethylformamide, dimethyl sulphoxide or acetonitrile. It is also possible to carry out the reaction in aqueous medium.
The reaction can also be carried out in the presence of an acid acceptor, such as, for example, a nitrogenous organic base, for example, triethylamine, an alkaline carbonate, for example, sodium carbonate, or an alkali metal or alkaline earth metal hydroxide.
According to the invention, the benzo[b][1,8]naphthyridine derivatives of general formula (I) can also be obtained from a corresponding ester of general formula (IV): 
in which R1, R2 and R4 are defined as above, R3 is defined as above or represents a protected alkylamino radical, and Alk represents an alkyl radical comprising 1 to 4 straight- or branched-chain carbon atoms, by any method known for producing an acid from an ester without affecting the remainder of the molecule.
The acid can be prepared from the ester by saponification in the presence of potassium hydroxide or sodium hydroxide, in aqueous or aqueous/alcoholic medium, at a temperature ranging from approximately 20 to approximately 100xc2x0 C.; it is also possible to carry out the reaction by acid hydrolysis at temperatures such as mentioned above.
When R3 represents a protected alkylamino radical, the protecting radical can be any amino-protecting group compatible with the molecule. It is possible to choose a protecting radical which can be removed simultaneously with the hydrolysis of the ester. Protection can be carried out by any compatible group, the use and the removal of which do not detrimentally affect the remainder of the molecule. The reaction can be carried out according to the methods described by T. W. Greene, (Protective Groups in Organic Synthesis, A. Wiley-Interscience Publication (1981)), or by McOmie, (Protective Groups in Organic Chemistry, Plenum Press (1973)), the relevant disclosure of each of which is hereby incorporated by reference.
The benzo[b][1,8]naphthyridine derivative of general formula (III) can be obtained by application of the methods disclosed in U.S. Pat. No. 4,990,515, or European patent No. EP 431,991 or EP 379,414, the relevant disclosure of each of which is hereby incorporated by reference, or by similar techniques.
The benzo[b][1,8]naphthyridine derivative of general formula (IV) can be obtained by reaction of an amine of general formula (II) with a corresponding ester of general formula (V): 
in which R3, R4, Hal and Alk are defined as above, according to the method described for the reaction of an amine of general formula (II) with the benzonaphthyridine derivative of general formula (III). It is understood that, in the alternative where the reaction is carried out in aqueous medium, it is possible to obtain a compound of general formula (I) directly without intermediate isolation of a derivative of general formula (IV).
The benzo[b][1,8]naphthyridine ester derivative of general formula (V) can be obtained as disclosed in European Patent No. EP 606,382, the relevant disclosure of which is incorporated herein by reference.
According to the invention, when stereoisomeric forms of the benzonaphthyridine derivatives of general formula (I) exist and when it is desired to obtain these stereoisomers, the stereoisomeric forms of the amines of general formula (II) can be separated by any known method compatible with the molecule. By way of example, the separation can be carried out by acylation with a chiral acid or a reactive derivative of a chiral acid, followed by separation of the isomers by high performance liquid chromatography, and then deacylation according to the method described by P. G. Gasseman et al., (J. Am. Chem. Soc., 98 (5), 1275 (1976)), the relevant disclosure of which is incorporated herein by reference. It is also possible to separate the stereoisomers by chiral phase high performance liquid chromatography.
The novel compounds according to the present invention and their synthetic intermediates can optionally be purified by physical methods, such as crystallization or chromatography.
The compounds according to the present invention and their intermediates of general formula (III) can be converted to metal salts or to addition salts with nitrogenous bases according to methods known per se. These salts can be obtained according to the usual methods, which do not detrimentally affect the remainder of the molecule, for example by the action of a metal base (e.g., alkali metal or alkaline earth metal base), ammonia or an amine of an abovementioned compound in an appropriate solvent, such as an alcohol, an ether or water, or by an exchange reaction with a salt of an organic acid. The salt formed precipitates after optional concentration of its solution and it is separated by filtration, sedimentation or lyophilization.
The novel compounds according to the invention can also be converted to addition salts with acids. The compounds of general formula (I) obtained in the form of these salts can be released and converted to salts of other acids according to the usual methods.
Mention may be made, as examples of pharmaceutically acceptable salts, of the salts with alkali metals (sodium, potassium or lithium) or with alkaline earth metals (magnesium or calcium), the ammonium salt, the salts of nitrogenous bases (ethanolamine, diethanolamine, trimethylamine, triethylamine, methylamine, propylamine, diisopropylamine, N,N-dimethylethanolamine, benzylamine, dicyclohexylamine, N-benzylphenethylamine, N,Nxe2x80x2-dibenzylethylenediamine, diphenylenediamine, benzhydrylamine, quinine, choline, arginine, lysine, leucine or dibenzylamine) and the addition salts with inorganic acids (hydrochlorides, hydrobromides, sulphates, nitrates or phosphates) or organic acids (succinates, fumarates, maleates, methanesulphonates, p-toluenesulphonates or isethionates).
The novel benzo[b][1,8]naphthyridine derivatives of general formula (I) according to the present invention and their pharmaceutically acceptable salts exhibit advantageous antibacterial properties. They display a remarkable in vitro and in vivo activity with regard to gram-positive microorganisms and with regard to microorganisms that are resistant to quinolones. Bearing in mind their activity, they are useful for topical administration.
In vitro, the compounds of general formula (I) have been shown to be active at a concentration ranging from about 0.06 to about 4 xcexcg/cm3 with regard to Staphylococcus aureus IP 8203 and at a concentration ranging from about 0.12 to about 4 xcexcg/cm3 with regard to Staphylococcus aureus N79, which is resistant to quinolones.
In vivo, the compound of Example 1 has been shown to be active at a concentration of about 2% in a cetomacrogol and benzyl alcohol formulation in the model of infection of the guinea-pig with Staphylococcus aureus ATCC 25923. It has also been shown to be active at about 2% with respect to the model of experimental infection of the guinea pig with Staphylococcus aureus AS 5155 strain, which is resistant to meticillin, MLSB and quinolones.
Finally, the compounds according to the invention do not exhibit toxicity at the doses used. After 2 weeks of topical application b.i.d. of a composition comprising the compound of Example 1 in the proportions of about 2, about 5 and about 10% to a rat, no toxic effect was observed. Likewise, after 2 weeks of topical application b.i.d. of a composition comprising the compound of Example 1 in the proportion of about 2% to the nasal mucous membrane of a rabbit, no toxic effect was observed either.
7-fluoro-8-[4-(4-fluorophenyl)piperazin-1-yl]-1-methyl-4-oxo-1,4-dihydrobenzo[b][1,8]naphthyridine-3-carboxylic acid and its salts are examples of useful compounds of general formula (I) in which:
R1 and R2, which are identical or different, represent an alkyl radical comprising 1 or 2 carbon atoms or a cyclopropyl radical, or form, together with the nitrogen atom to which they are attached, a 6-membered heterocycle, optionally comprising another nitrogen atom, which can be substituted at the 4-position by a phenyl radical, a phenyl radical substituted by a halogen atom or by a methylthiomethyl radical, a benzyl radical, a benzyl radical substituted by a halogen atom, a heterocyclylmethyl radical, the heterocyclyl part of which is unsaturated and comprises 5 members, or a phenylamino radical, the phenyl part of which is optionally substituted by a halogen atom, or R1 and R2 form, together with the nitrogen atom to which they are attached, a 7- or 8-membered heterocycle;
R3 represents an alkyl radical comprising 1 or 2 carbon atoms, a cyclopropyl radical, a difluorophenyl radical or a methylamino radical; and
R4 represents a hydrogen atom or a fluorine atom; and their salts.
The following examples, given without implied limitation, illustrate the present invention.